Many different types of printing have been invented, a large number of which are presently in use. The known forms of printing have a variety of methods for marking the print media with a relevant marking media. Commonly used forms of printing include offset printing, laser printing and copying devices, dot matrix type impact printers, thermal paper printers, film recorders, thermal wax printers, dye sublimation printers and ink jet printers both of the drop on demand and continuous flow type. Each type of printer has its own advantages and problems when considering cost, speed, quality, reliability, simplicity of construction and operation etc.
In recent years, the field of ink jet printing, wherein each individual pixel of ink is derived from one or more ink nozzles has become increasingly popular primarily due to its inexpensive and versatile nature.
Many different techniques on ink jet printing have been invented. For a survey of the field, reference is made to an article by J Moore, “Non-Impact Printing: Introduction and Historical Perspective”, Output Hard Copy Devices, Editors R Dubeck and S Sherr, pages 207-220 (1988).
Ink Jet printers themselves come in many different types. The utilisation of a continuous stream of ink in ink jet printing appears to date back to at least 1929 wherein U.S. Pat. No. 1,941,001 by Hansell discloses a simple form of continuous stream electro-static ink jet printing.
U.S. Pat. No. 3,596,275 by Sweet also discloses a process of a continuous ink jet printing including the step wherein the ink jet stream is modulated by a high frequency electrostatic field so as to cause drop separation. This technique is still utilized by several manufacturers including Elmjet and Scitex (see also U.S. Pat. No. 3,373,437 by Sweet et al).
Piezoelectric ink jet printers are also one form of commonly utilized ink jet printing device. Piezoelectric systems are disclosed by Kyser et. al. in U.S. Pat. No. 3,946,398 (1970) which utilizes a diaphragm mode of operation, by Zolten in U.S. Pat. No. 3,683,212 (1970) which discloses a squeeze mode of operation of a piezoelectric crystal, Stemme in U.S. Pat. No. 3,747,120 (1972) discloses a bend mode of piezoelectric operation, Howkins in U.S. Pat. No. 4,459,601 discloses a piezoelectric push mode actuation of the ink jet stream and Fischbeck in U.S. Pat. No. 4,584,590 which discloses a shear mode type of piezoelectric transducer element.
Recently, thermal ink jet printing has become an extremely popular form of ink jet printing. The ink jet printing techniques include those disclosed by Endo et al in GB 2007162 (1979) and Vaught et al in U.S. Pat. No. 4,490,728. Both the aforementioned references disclose ink jet printing techniques that rely upon the activation of an electro-thermal actuator which results in the creation of a bubble in a constricted space, such as a nozzle, which thereby causes the ejection of ink from an aperture connected to the confined space onto a relevant print media. Manufacturers such as Canon and Hewlett Packard manufacture printing devices utilizing the electro-thermal actuator.
A particular problem associated with thermal printers is that they are not suitable for pagewidth printheads capable of high definition printing. Such printheads require a very large number of densely packed nozzle arrangements that span a print medium. Applicant submits that operation of a required number of electro-thermal actuators would generate an unacceptable level of heat. This is the primary reason why the Applicant has developed MEMS-based printing technology. Such systems can be fabricated to define printhead chips having a large number of densely packed nozzle arrangements. In particular, Applicant has developed page width printheads that are capable of color printing over 20 pages per minute at resolution finer than 1200 dpi. Applicant has carried out many thousands of simulations in order to achieve an optimal design. In this work, Applicant has found that the ink pusher should move at least 1 micron in order to achieve effective drop ejection. Applicant has also found that certain problems arise with fabrication when an ink pusher is designed to move more than 5 microns. It follows that movement between 1 and 5 microns is most preferable.
As set out above, piezoelectric systems have been developed that act physically on the ink to eject the ink from the nozzle arrangements. In order to achieve ink drop ejection, the ink pusher is required to move through a particular range. Piezo-electric systems that use an ink pusher rely on the deflection of a plate or the like as a result of a force exerted in a direction that is generally at right angles to the direction of drop ejection. Applicant has found that a deflection of at least one micron would require a plate having a cross sectional area in excess of 100 square microns. This requirement precludes the fabrication of a printhead chip having the requisite number and density of nozzle arrangements.
A further problem with piezoelectric systems is that it is difficult to achieve an ink pusher that is capable of more than 100 nanometers of movement. This is largely due to the fact that such systems rely on buckling or deflection to achieve the necessary movement.
When creating a large number of inkjet nozzles which together form a printhead, it is necessary or desirable to ensure that the printhead is of a compact form so as to ensure that the printhead takes up as small a space as possible. Further, it is desirable that any construction of a printhead is as simple as possible and preferably; the number of steps in construction is extremely low, therefore ensuring simplicity of manufacture. Further, preferably each ink ejection nozzle is of a standard size and the ink forces associates with the ejection are regular across the nozzle.
Further, where the ink ejection mechanism is of a mechanical type attached to an actuator device, it is important to ensure that a substantial clearance is provided between an ink ejection nozzle and the surface of the paddle. Unless a large clearance is provided (of the order of 10 microns in the case of a 40 micron nozzle) a number of consequential problems may arise. For example, if a mechanical paddle ejection surface and nozzle chamber walls are too close, insufficient ink will be acted on by the paddle actuator so as to form a drop to be ejected.
Further, high pressures and drag is likely to occur where movement of a paddle occurs close to nozzle chamber walls. Further, if the paddle is too close to the nozzle, there is a danger that an unwanted meniscus shape may occur after ejection of an ink drop with the ink meniscus surface attaching to the surface of the paddle.
Further, should the ink ejection mechanism be formed on a silicon wafer type device utilizing standard wafer processing techniques, it is desirable to minimize the thickness of any layer of material when forming the system. Due to differential thermal expansions, it is desirable to ensure each layer is of minimal thickness so as to reduce the likelihood of faults occurring during the fabrication of a printhead system due to thermal stress. Hence, it is desirable to construct a printhead system utilizing thin layers in the construction process.
This invention is based on the fact that the Applicant has achieved a generic MEMS structure that facilitates a particular range of movement of an ink pusher, which can be in the form of a paddle. The advantage of this is set out above.